How to drive object's local transform by another object, while relative to third object?
You can do this with drivers and parenting.

Use Rightclick &gt; Copy New Driver and then RMB &gt; Paste Driver
Drive all location and rotation fields from Camera A to Camera B  (so they match perfectly)
Select Camera B and go Object Properties &gt; Relations &gt; Parent and target Object B


2. RMB &gt; Copy Driver &gt; Paste Driver from Camera A to Camera B. Do this for each location/rotation field. 


3. In Object Properties of Camera B, target Object B.
Camera B will now inherit Object B rotation/location, but will also keep the driven position and rotation on top of that.